1. Field of the Invention
This invention relates to a transfer differential gear lock controller in full-time four-wheel drive vehicles.
2. Description of the Prior Art
Generally, when a vehicle turns a curve, a running distance of outer wheels is greater than a running distance of inner wheels. Therefore, two-wheel drive vehicles such as FF types and FR types are equipped with a differential gear to provide the driving wheels with the difference of rotation between the inside and the outside so that the vehicles can smoothly turn the curve. If a tire or tires on one side completely slip in the mud or the like, the tire or tires run merely idle and driving force is consumed only by the idle tire or tires but is not transmitted to the tire(s) which firmly grips the ground. To cope with this problem, vehicles are generally equipped with a mechanism which stops temporarily the function of the differential gear and transmits the driving force also to the tire gripping the ground or in other words, a differential gear lock mechanism, when the tire or tires on only one side turn idle excessively. In the full-time four-wheel drive vehicles, however, they must have a structure such that the four wheels generate the differences of rotation between them. Therefore, a device for distributing the driving force of an engine to the front and the back, that is, a center differential gear as a third differential gear is provided besides a front differential gear disposed between the front wheels and a rear differential gear disposed between the rear wheels. Furthermore, a center differential gear lock mechanism is provided to the center differential gear in order to prevent part of the tires from running idle.
An example of the conventional full-time four-wheel drive vehicles is disclosed, for example, in the specification and drawings of U.S. Pat. No. 3,848,691. In this full-time four-wheel drive vehicle, the driving force is inputted via a sub-transmission gear to a bevel gear type center differential gear disposed at the rear part of a transfer through a main shaft, the output of a front driving system is driven through a chain by a sprocket disposed at the front part of the center differential gear so as to transmit a torque to the front wheels and the output of the other rear driving system transmits a torque to the rear wheels via side gears of the center differential gear. A center differential gear lock mechanism of a spline-fitting type is disposed between the center differential gear and the sprocket. This center differential gear lcok mechanism is formed by providing a mechanical lock to the center differential gear. The mechanical differential lock is operated by a control lever or an actuator linked from the transfer.
U.S. Pat. No. 3,923,113 discloses a four-wheel drive vehicle in its specification and drawings. This four-wheel drive vehicle includes a viscous type differential limiter at the outer peripheral portion of a bevel gear type center differential gear which has the same function as the bevel gear type center differential gear in the full-time four-wheel vehicle described above.
In conjunction with the full-time transfer apparatus described above, those vehicles which are directed primarily to off-road driving are generally equipped with a high and low two speed type switching device or a two-range switching device for switching the speed change range to either a high speed range or a low speed range. In the full-time four-wheel drive vehicles in general, the differential gear lock must be brought into an operative state under the road surface condition where the vehicles must drive in the low range, in the case of the transfer equipped with the center differential gear. In such full-time four-wheel drive vehicles, it is necessary for a driver to conduct two kinds of operations, that is, the operation for switching the transmission from the high range to the low range and then the operation for operating the differential gear lock so as to lock the center differential lock mechanism.
In the bevel gear type center differential gear in the full-time four-wheel drive vehicles of the prior art described above, the torque transmitted to the main shaft of the transmission is transmitted to the center differential gear at a torque transmission ratio of 1:1 because the center differential gear is positioned at the rear part of the transfer. Particularly, the torque reduced in the low range is transmitted as it is. In such a case, a somewhat great torque is applied to the main shaft. Therefore, to secure the strength of the main shaft, any measures such as the increase of the diameter of the main shaft must be taken, thereby inviting the problems such as the increase in the weight of the main shaft and its cost of production.
In the full-time four-wheel drive vehicles described above, one of the sleeves of the center differential lock is connected to the side gear on the front driving side and the other, to a differential gear carrier of the center differential gear. Therefore, when the differential gear lock mechanism operates to attain the lock state, the torque transmission route becomes the front wheels.fwdarw.the front driving system.fwdarw.the sleeve of the differential gear lock mechanism.fwdarw.the differential gear carrier.fwdarw.the rear driving system.fwdarw.the rear wheels, and the tooth surfaces of the pinion of the center differential gear and of the side gears always receive the torque. In order to secure durability of the side gears and pinion of the center differential gear, therefore, the sizes of these gears must be increased to improve strength, and this disadvantageously increases cost, weight, space and so forth.
In connection with the structure of the center differential gear, the pinion is fixed to the differential gear carrier but the side gear of the front driving system and the side gear of the rear driving system are not housed inside the differential gear carrier. Accordingly, the back lash occurring between the pinion and the side gears of the center differential gear results in the occurrence of abnormal sound during driving such as the gear striking sound due to the back lash of the bevel gear or the pinion in the case of the full-time four-wheel drive vehicles. Though the control of the back lash is indispensable in such a center differential gear, repetition of the total disassembly and assembly of the transfer portion becomes necessary at the time of selection of washers and the like that are disposed as measures for each side gear thrust surface, makes the assembly more troublesome and is not desirable from the aspect of productivity.
In the four-wheel drive vehicles described above, a viscous coupling is disposed at the outer peripheral portion of the bevel gear type center differential gear. Accordingly, in addition to the problems with the full-time four-wheel drive vehicles described above, this invites another problem that the dimension in the radial direction becomes greater, though the space in an axial direction can be reduced advantageously, and this presents a serious problem particularly in the case of a front-engine rear-drive type vehicle because the space between it and a floor tunnel is limited.